This invention relates to a piezoelectric cable suitable for use in fish finding or detection of marine seismic conditions and to a method of unwinding a wound cable.
It is known to utilize a piezoelectric cable for finding fish or for detecting marine seismic conditions. Illustrated in FIG. 11 is one known piezoelectric cable 30' which includes a core electrode 31, a piezoelectric layer 32 surrounding the core electrode 31, and an outer electrode 33 provided to surround the piezoelectric layer 32. The piezoelectric layer 32 is polarized by impressing a direct voltage across the core and outer electrodes 31 and 33. In use, one end of the cable 30' is connected to a winch mounted on a boat. The cable 30' is trailed by the boat with the other end thereof being rendered free. Acoustic waves propagating through sea water are thus detected by the piezoelectric layer and the detected signals are outputted from terminals 34 leading from the core and outer electrodes 31 and 33.
As the piezoelectric layer 32, an organic piezoelectric material such as a polyvinylidene fluoride, a polyvinyl fluoride, a polyvinylidene chloride, a polyvinyl chloride and a polyamide, or a composite-type piezoelectric material containing finely divided ferroelectric ceramic such as lead titanate or lead titanate zirconate dispersed in an organic matrix such as a synthetic rubber or a synthetic resin is used because the acoustic impedance of such flexible piezoelectric materials resembles that of water.
The known piezoelectric cable, however, suffers from a drawback that, as shown in FIG. 12, when the cable 30' wound around a winch 35 on a boat 36 is unwound for immersion into sea water, it is curled or waved. Such curls cause cavitation or random flows thereabout when the cable is trailed so that undesirable signals generally called "flow noises" are generated.